Heat a large deep skillet over medium-high heat. Add 1 tbs of the oil and the mushrooms. Cook the mushrooms, shaking or stirring frequently until they begin to brown. Add the onion and cook for about 2 minutes or until they turn translucent. Remove mushrooms and onions to a plate.
Add the remaining 1 tbs of oil and pat the meat dry with paper towels. When the oil is hot, add the meat to the pan to brown. You may have to do this in shifts--don't overcrowd the pan as the meat won't brown if you do. Remove the meat to a plate.
Return the pan to the heat and deglaze with the beer, scraping up all the bits at the bottom of the pan.
Add the stock, soy sauce, thyme, garlic powder, salt and pepper. Stir to combine.
Add the meat, mushrooms and onions back to the pan. Bring to boil, cover and reduce heat and simmer for 1 hour or until meat is tender.
Mix the flour and water so there are no lumps. Stir into the meat mixture continuing to stir until liquid is thickened and it bubbles. If the liquid seems too light, add the Gravy Master.
Preheat the oven to 425F/220C. Place the meat in a 1.5 quart/1.5 liter round baking dish or pie pan.
Cut an 8-inch/20 cm circle of the puff pastry. For a nice effect, score the pastry with diagonal lines, being careful not to cut through the pastry (nice but not necessary).
Place the pastry circle on top of the meat and brush with beaten egg yolk.
Bake for 15-20 minutes or until crust is puffed and golden brown.